Typical beam expanding telescopes are refractive involving a negative input lens in line with a positive objective lens; or are reflective involving a concave primary or input mirror reflecting to a concave secondary mirror. In these cases, the ratio of focal lengths defines the power or magnification of the telescope. Such telescopes are typically mounted in metal housings, precision aligned, and sealed against environment. The problem with these prior telescopes is that they include alignment features or mechanisms adding weight and volume, and are prone to misalignment from outside sources. Thus, the disadvantages of such telescopes are that they are larger, less inherently stable, and subject to contamination. This is of concern when the telescopes are used in harsh military environments.
Confronted with optical performance requirements vs. size/space allocations on a recent gimbal mounted military laser target designator countermeasure system, a new telescope needed to be designed which would enhance optical performance and system stability. The configuration would have to be the smallest, most compact telescope of comparable performance one could build. It is noted that conventional afocal, no common focus, telescopes consist of two parabolic surfaces with a common axis of rotation, but with different focal lengths, spaced and supported by stable structures. Just as the power of a sighting telescope describes image magnification mentioned above, the power of the telescope when used to project laser energy describes reduction of laser beam divergence to provide a tighter, smaller beam on the intended target. Beam divergence is reduced by the power of the telescope, and energy on target increases by the square of that power.
There is therefore a requirement to provide a stable miniaturized telescope that will fit into the gimbal. Ordinarily when one, for instance, seeks to have an 8 power telescope. As described above, one would use refracting optics with a negative lens and expand the beam to a positive lens, with the ratio of the apertures defining the power out of the telescope. Thus, if one requires an 8 power telescope in order to get the beam focused for long range applications, one needs to design an 8 power telescope to fit into the tightly packaged gimbal. Size, weight, and alignment are the important design considerations, just as temperature, shock, and vibration are the important environmental concerns.
It will be appreciated that most telescopes have multiple elements which creates a variety of alignment problems including spacing, temperature coefficients of expansion, vibration mounting and the like. In order to design such a telescope one has to match coefficient of expansion of the housing to the elements so that one doesn't move one element relative to the other and therefore defocus or misalign the telescope.
It will also be appreciated that mounting and aligning multiple elements such as negative and positive lenses takes up a fair amount of space, regardless of whether or not environmental problems can be solved.
Therefore in laser target designators and other gimbal mounted laser systems, there is a requirement for a telescope that is extremely compact and immune to environmental factors which can cause optical aberrations and unwanted beam divergence, and which also can contribute to aiming errors.